1. Field of the Invention
The present invention relates to a method for forming a semiconductor device. More particularly. the present invention relates to a method for forming a shallow trench isolation structure.
2. Description of the Related Art
An isolation region is formed in an integrated circuit for preventing a short circuit from occurring between adjacent device regions on a substrate. Conventionally, a local oxidation of silicon (LOCOS) technique is widely utilized in the semiconductor industry to provide isolation regions on semiconductor device. However, since internal stress is generated and bird's beak encroachment occurs in the isolation structures. LOCOS cannot effectively isolate devices.
The shallow trench isolation (STI) technique has been developed to improve the bird's beak encroachment of the LOCOS so as to achieve an effective isolation structure. Typically, the STI process comprises the steps of using a mask to define and pattern a shallow trench on a substrate by anisotropic etching process, and then filling the shallow trench with oxide for use as a device isolation structure.
Conventionally, the sidewalls of the shallow trench isolation structure are usually perpendicular to the substrate surface; that is, the corner of the shallow trench isolation structure is a right angle. As a result, high stress occurs at the corner of the shallow trench isolation structure because the corner is a right angle. The stress leads to damage of the silicon lattices in the substrate surface. In a subsequent step, a gate oxide layer formed on the substrate is not able to form a predetermined thickness because the silicon lattices of the substrate surface are damaged. Therefore, a leakage current occurs at the corner of the shallow trench isolation structure. Many methods have been proposed to solve this problem; one method is that a patterned photoresist layer for defining the shallow trench be partially removed by a plasma ashing process, and then a portion of the substrate be removed by oxygen plasma etching to form the shallow trench. With sloped photoresist, the shallow trench will also has sloped profile due to pattern transfer effect during plasma etching. The polymer flow down the sidewalls of the shallow trench to protect the sidewalls of the shallow trench plasma etching. Therefore, the sloped shallow trench sidewall is formed by plasma ashing the photoresist layer after the photoresist layer has been exposed and developed. A width of the shallow trench is gradually decreased from top to bottom.
Since the sidewalls of the shallow trench and the substrate surface intersect at an obtuse angle; that is, the corner of the shallow trench formed by the method mentioned above is an obtuse angle. The stress is not easily generated and leakage current of the shallow trench isolation structure is avoided. However, the etching rate of the photoresist layer removed by oxygen plasma is hard to control, so that it is difficult to protect the sidewalls of the shallow trench by using the polymer formed from the photoresist layer. If the etching rate is too fast, the sidewalls of the shallow trench are easily etched by the oxygen plasma. Therefore, an angle between the substrate surface and the sidewalls of the shallow trench is decreased. If the etching rate is too slow, the shallow trench can not effectively isolate devices.